To ensure the reliability of an electric power grid, the administrator must continually maintain a power reserve in order to compensate for a possible failure of energy production units. The power reserve is essentially an excess production capacity on standby. In normal conditions, the power generation units are run at less than 100% such that a degree of reserve power is always available. However, the maintenance of this reserve capacity is an expensive proposition since the reserve constitutes a resource that cannot be effectively monetized by the utility company.
An electric power grid will operate in normal conditions at a fixed frequency (usually 50 or 60 Hz). The frequency remains constant as long as the supplied power matches the power consumed by the load. Any sudden changes in generation or load resulting in an unbalance between generation and load will lead to a frequency instability during which the frequency deviates from its nominal value. Large frequency variations are undesirable because they could lead to equipment trip or even a system collapse.
Frequency instability events are generally caused by the sudden loss of a power generation unit or by the loss of a large load and are characterized by a sudden frequency variation from the frequency nominal value.
The reserve capacity in a power grid is thus tapped when the frequency drops below a certain level. Electrical generation units that supply power to the grid are equipped with a speed governor. The speed governor continuously regulates the power output of generation units in order to balance the generation with the load. Thus when the frequency of the grid varies, the speed governor responds to this variation to compensate it. For example, when the frequency is higher than normal, the speed governor will simply lower the power generated by the generation unit (therefore reducing the amount of power supplied to the grid). Alternatively, when the frequency is lower than normal, the speed governor will increase the power generation. The speed governor however has some inherent limitations. In particular, it is slow to respond since it involves certain mechanical constraints. Depending of the type of generation (hydraulic, gas, thermal, wind, etc . . . ) some time is required for the generation unit to increase its speed up to the desired point.
System inertia is another aspect to frequency stability of the power grid. “Inertia” refers to the ability of the grid to buffer energy imbalances, such as excess load or excess generation and thus prevent significant and rapid frequency variations. Any power grid has a level of inherent inertia on its generation side. This inherent inertia is in the form of mechanical energy stored in the rotors of the generators. If the load on the power grid increases, the rotor inertia of a generator will be able to instantly respond to this increased load and thus dampen a frequency drop. Similarly, if the load connected to the grid is suddenly reduced, the rotor inertia will limit its tendency to overspeed, hence increase the frequency of the supply voltage.
Accordingly, from the perspective of frequency stability, some level of inertia in the power grid is desirable because it acts as a mechanism to dampen frequency variations and thus provides more time for slower frequency stabilization systems to become active.